1. Field of the Invention
This invention relates to a device for detecting a focus in an optical instrument, and more particularly to a device for detecting the position of an objective of an optical instrument where the image of an object is focused on a plane where a sharp image is desired to be focused. This invention is particularly applicable to a photographic camera for automatically focusing an image on a film in the camera.
2. Description of the Prior Art
It has been known in the art to detect a focus in an optical instrument by use of a vibrating prism or the like. For instance, the basic principle of the automatic focusing method using a vibrating prism is disclosed in The British Journal of Photography, July 1975, P.594-596 and also mentioned in copending U.S. patent applications (Ser. No. 750,005 now U.S. Pat. Nos. 4,114,994 and 754,699) now U.S. Pat. No. 4,090,209, filed Dec. 13 and 27, 1976.
The vibrating prism or the like is periodically vibrated in a direction perpendicular to the optical axis of an objective substantially on the focal plane of the objective. When the image focused by the objective is accurately focused on the vibrating prism or the like, the image viewed from the back of the prism or the like stands still. When the image is not accurately focused on the prism, the image viewed from the back of the prism or the like vibrates. The vibration of the image is electrically detected by a means for detecting vibration of an image. The means for detecting vibration of an image uses an electric circuit for detecting an alternating current component of an output of a photodetector. The alternating current component taken out by the circuit is amplified and rectified.
The above described focus detecting device is advantageous in that the focus is detected with high response even in case of a dark and/or moving subject matter. This device is, however, disadvantageous in that the correct focus detection cannot be expected when a subject matter having high contrast exists, in the image, in the vicinity of the principal subject matter (mainly a figure). This is because the photodetector used for receiving light from the principal subject matter has a definite size which will possibly also cover the non-principal subject matter existing in the vicinity of the principal subject matter. If the non-principal subject matter is at the same distance as the principal subject matter, there is no problem. However, if the non-principal subject matter and the principal subject matter are at the different distances, there is a fear of detecting the focus with respect to the non-principal subject matter.